Hi,
I have a thermosyhon air heating collector on my barn/shop. I like it a lot -- no fans or controls. The air exits vents near the top of the collector on the wall up close to the ceiling. This area runs quite hot when the collector is running -- the vent exit temperature runs up toward 120F with good flow. I have thought about placing some thermal mass in this area. Just as a test, I've put a gallon can up there filled with water, and it heats up well over the collection period. The challenge would be getting enough mass in place to be effective.
Perhaps a plastic lined trough filled with water and hung from the ceiling? (you did ask for all sorts of ideas
Or, maybe even pick up heat from this area with a car radiator as Nathan does in this scheme:http://www.builditsolar.com/Projects...rageNathan.pdf
The radiator could transfer the heat to a water tank -- if the tank was centrally located in the garage, and not insulated, it could act like a big radiator?

I guess another approach would be to 1) insulate well, 2) add something like the large area thermosyphon collector (lots of collection area), 3) add distributed thermal mass throughout the garage -- water barrels, or anything with lots of mass, and 5) keep the air well mixed (maybe a slow, efficient ceiling fan or two). This would allow the excess of heat collected during the day to be absorbed by thermal mass (which should help prevent overheating), and the good insulation should allow the mass to carry over the heat to the next morning.

I'm sure you already know about the Solar Site Survey -- avoids those "how can I be such an idiot" moments that we all have Solar Site Survey